Hell....I dont think I've ever seen my battery lower than about 85, it's 91 right now in the a/c house, and I've seen it go as high as 107 ! It usually only gets hot when I first start streaming music at work in the mornings, but I think I've figured out if I close the app and start it again, it wont do that. It gets hot using navigation of course. For me, hard to fight it when it is 100 degrees here anyway and I am outside a lot.

Any sort of heavy use can create heat (data, CPU, etc) ... some due to the various wireless radios and some heat due to the heavier battery drain. I have noticed the 4G radio can make it run a bit more hot in general even without heavy traffic as well. So if everything is turned on (GPS, 4G, WiFi, Bluetooth) I'm sure that can cause a higher average temp, not counting any background apps that may be doing frequent data syncs.

As far as your temps though, I wouldn't worry about it. At home (72-75 air temp) mine ranges from 80 to 100 depending on what I'm doing, typically with GPS and WiFi radios on, 4G/Bluetooth off.

Until you pass up 115+ I don't think there is much of an issue. It has been 100+ degrees outside (110-115 heat index) where I live for the past week, and my phone pushes past 125 degrees while I'm in the car (A/C is broken and dark interior, so my car temp is always higher than outside air even with windows open). After about 120 Battery Indicator (3rd party app) shows "overheat" and the phone stops trying to charge, I have just been shutting it off after that. Hopefully after today the car temps shouldn't be as much of an issue for a while since the high temps should be no more than 90 next week, but I'll still be paying attention to see if it peaks past 120 if I'm running GPS or heavy data while it is in the car.

My phone got to 120 degrees the other day. I was streaming Pandora through the aux input while plugged into the car charger. And every day at work, I am plugged into the outlet charging and streaming Pandora. Right now the battery is 105.

Sometime if I quit the app and re-start, the battery will go back to "normal".

So is it better to not charge while streaming Music, is that the main problem? Not streaming the music at work or in the car is not an option

Battery temps are one of the few measurements I prefer metric. Not sure why. I guess in school when I was taking heat transfer class, we only talked about chip temps in C. Anyhow: 100F is a very safe temperature for a Li-ion battery. Try to keep it below 120F. I know prolonged use of the GPS can raise temps quite a bit.